Beam homogenizer and laser irradiation apparatus

ABSTRACT

The present invention provides a beam homogenizer for homogenizing energy distribution by making the distance between lenses small to shorten the optical path length with the use of an array lens of an optical path shortened type, and a laser irradiation apparatus using the beam homogenizer. The beam homogenizer is equipped with a front side array lens of an optical path shortened type whose second principal point is positioned ahead on a beam incidence side, a back side array lens of an optical path shortened type whose first principal point is positioned behind on a beam emission side, and a condensing lens, wherein the distance between the second principal point of the front side array lens and the first principal point of the back side array lens is equal to the focal length of the back side array lens.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a beam homogenizer which homogenizesthe energy distribution of a beam spot on an irradiation surface byusing a more compact optical system, and also relates to a laserirradiation apparatus using the beam homogenizer. More specifically, thepresent invention relates to a beam homogenizer which homogenizes theenergy distribution of a beam spot on an irradiation surface by using amore compact optical system obtained by decreasing the distance betweenlenses to shorten optical path length, and also relates to a laserirradiation apparatus using the beam homogenizer.

2. Description of the Related Art

In recent years, research has been widely conducted on techniques forcarrying out laser annealing to a non-single crystal semiconductor film(an amorphous semiconductor film or a semiconductor film havingcrystallinity which is not single crystal, such as poly-crystal ormicrocrystal) formed over an insulating substrate such as a glasssubstrate. The laser annealing described herein indicates a techniquefor recrystallizing an amorphous layer or a damaged layer formed in asemiconductor substrate or a semiconductor film, and a technique forcrystallizing a non-single crystal semiconductor film formed over asubstrate. Moreover, the laser annealing includes a technique applied tomodification or flattening of a surface of a semiconductor substrate ora semiconductor film, a technique in which laser irradiation is carriedout after introducing a crystallization-promoting element such as nickelinto an amorphous semiconductor film, a technique in which asemiconductor film having crystallinity is irradiated with a laser beam,and so on.

The laser annealing is employed for the crystallization because a glasssubstrate has a low melting point and a glass substrate is deformed whensubstrate temperature gets too high at the annealing. In contrast, alaser can give high energy only to a non-single crystal semiconductorfilm without changing the temperature of a substrate that much. As alaser annealing method, the following method is often employed becauseof its high mass productivity and industrial superiority: a laser beamemitted from a pulsed laser with high output power such as an excimerlaser is shaped into a square spot with several centimeters on each sideor into a rectangular spot with a length of 10 cm or more in a long-sidedirection on an irradiation surface through an optical system and anirradiation position of the beam spot is moved relative to theirradiation surface. It is to be noted that among the rectangular beamspots, a rectangular beam spot having a particularly high aspect ratiois referred to as a linear beam spot in this specification.

In particular, unlike a punctate beam spot requiring to be moved fromfront to back and from side to side, a linear beam spot can provide highmass productivity because a large irradiation surface can be irradiatedwith the linear beam spot only by moving the linear beam spot in onedirection perpendicular to a direction along a longer width of thelinear beam spot. The laser beam is moved in the direction (hereinafterreferred to as a short-side direction) that is perpendicular to thedirection (hereinafter referred to as a long-side direction) along thelonger width of the linear beam spot since this is the most effectivescanning direction. Because of such high mass productivity, currentlaser annealing is mainly employing a linear beam spot obtained byshaping a beam spot of a pulsed excimer laser through an appropriateoptical system.

FIGS. 6A and 6B show an example of an optical system for changing asectional shape of a beam spot into a linear shape on an irradiatedsurface. The optical system shown in FIGS. 6A and 6B is an extremelygeneral optical system. This optical system not only changes thesectional shape of the beam spot into a linear shape but alsohomogenizes the energy distribution of the beam spot on the irradiationsurface at the same time. Generally, the optical system for homogenizingthe energy distribution of the beam spot is referred to as a beamhomogenizer.

The optical system shown in FIGS. 6A and 6B is also a beam homogenizer.In the case of using a XeCl excimer laser (wavelength 308 nm) as a lightsource, it is preferable to use quartz as a base material of the opticalsystem. If another excimer laser with a shorter wavelength is used as alight source, it is preferable to use fluorite, MgF₂, or the like as thebase material in order to obtain high transmittance.

FIG. 6A is a side view of a beam homogenizer for forming a linear beamspot. The side view includes in its paper a short-side direction of thelinear beam spot formed by the beam homogenizer. A spot of a laser beamemitted from a laser oscillator 601, which is a XeCl excimer laser, isdivided in one direction by cylindrical lens arrays 602 a and 602 b. Ifa mirror is inserted in the middle of the optical system, the short-sidedirection is bent in a direction of light bent by the mirror.

In this structure, the beam spot is divided into four beam spots. Thesespots are combined into one spot once by a cylindrical lens 604. Thebeam spots that have separated again are reflected on a mirror 606 andthen condensed into one beam spot again by a doublet cylindrical lens607 on an irradiation surface 608. The doublet cylindrical lens is alens including two cylindrical lenses. This homogenizes the energy ofthe linear beam spot in its short-side direction and determines thelength of the linear beam spot in its short-side direction.

FIG. 6B is a plan view of the beam homogenizer for forming the linearbeam spot. The plan view includes in its paper a long-side direction ofthe linear beam spot formed by the beam homogenizer. The spot of thelaser beam emitted from the laser oscillator 601 is divided in adirection perpendicular to the long-side direction by a cylindrical lensarray 603. If a mirror is inserted in the middle of the optical system,the long-side direction is bent in a direction of light bent by themirror.

In the structure of this lens array 603, the beam spot is divided intoseven beam spots. After that, the seven beam spots are combined into onespot by a cylindrical lens 605 on the irradiation surface 608. Anoptical path after the mirror 606 is shown with dotted lines, andmoreover the dotted lines show correct optical path and position of theirradiation surface in the case of not providing the mirror 606. Thishomogenizes the energy of the linear beam spot in its long-sidedirection and determines the length of the linear beam spot in itslong-side direction.

As mentioned above, the cylindrical lens array 602 a and the cylindricallens array 602 b and the cylindrical lens array 603 serve as lenses fordividing the spot of the laser beam. The homogeneity of the energydistribution of the obtained linear beam spot is determined based on thenumber of the divided beam spots.

In general, an excimer laser emits a rectangular laser beam having anaspect ratio of approximately 1 to 5. The spot of the laser beam hasGaussian intensity distribution where the intensity is higher toward thecenter of the spot. The optical system shown in FIGS. 6A and 6B changesthe beam spot into a linear beam spot having homogeneous energydistribution with a size of 320 mm×0.4 mm.

The linear beam spot shaped by the above structure is delivered as beingoverlapped in such a way that the linear beam spot is displacedgradually in the short-side direction of the linear beam spot. Then, thelaser annealing can be carried out to the whole surface of a non-singlecrystal semiconductor film so as to crystallize it or to enhance itscrystallinity. In mass-production factories, currently, laser annealingis performed to semiconductor films using a linear beam spot shaped bythe optical system as above. It is to be noted that some beamhomogenizers include a reflecting mirror (for example, see PatentDocument 1: Japanese Published Patent Application Laid-Open No.2001-291681).

For several years, the size of a glass substrate has increased rapidlyin a semiconductor device manufacturing process in order to form moresemiconductor devices with one substrate and to improve massproductivity. With the increase in the size of the glass substrate, theimprovement of laser annealing process capability by means of theextension of a linear beam in its long-side direction has been morestrongly demanded. However, the extension of a linear beam in itslong-side direction causes a problem in that the size of an opticalsystem for forming the linear beam increases with the extension of thelinear beam in its long-side direction, thereby increasing the areaoccupied by the optical system.

SUMMARY OF THE INVENTION

The present inventor has perfected the present invention as the resultof concerted study directed towards eliminating this problem to providea compact beam homogenizer for forming a rectangular beam, particularlya linear beam. Therefore, it is an object of the present invention toprovide a beam homogenizer for homogenizing the energy distribution of abeam spot on an irradiation surface by using a more compact opticalsystem, i.e., an optical system in which optical path length isshortened by decreasing the distance between lenses, and to provide alaser irradiation apparatus using the beam homogenizer.

As described above, the present invention is to provide a compact beamhomogenizer for forming a rectangular beam, particularly a linear beam,and a laser irradiation apparatus using the beam homogenizer. The beamhomogenizer according to the present invention uses an array lens of anoptical path shortened type in which a principal point is set so thatthe optical path can be shortened and the beam homogenizer of thepresent invention can be roughly categorized into the following threemodes according to its usage: a first mode where array lenses of anoptical path shortened type are used as both array lenses at a frontside and a back side, a second mode where an array lens of an opticalpath shortened type is used as only an array lens at a front side, and athird mode where an array lens of an optical path shortened type is usedas only an array lens at a back side.

The array lens used in the present invention is an aggregation of lensesformed by connecting a plurality of small lenses and beams that havepassed through the respective small lenses focus at the same position bypassing through a condensing lens. The array lens is a generic term fora cylindrical lens array, a fly-eye lens, a crossed cylindrical lensarray, and the like. The cylindrical lens array is sometimes referred toas a cylindrical array lens.

The respective beam homogenizers of the present invention arespecifically shown hereinafter. A beam homogenizer of the first modeaccording to the present invention is equipped with a front side arraylens of an optical path shortened type whose second principal point ispositioned ahead on a beam incidence side, a back side array lens of anoptical path shortened type whose first principal point is positionedbehind on a beam emission side, and a condensing lens, wherein thedistance between the second principal point of the front side array lensand the first principal point of the back side array lens is equal tothe focal length of the back side array lens.

A beam homogenizer of the second mode according to the present inventionis equipped with a front side array lens of an optical path shortenedtype whose second principal point is positioned ahead on a beamincidence side, a back side array lens of an optical path unshortenedtype, and a condensing lens, wherein the distance between the secondprincipal point of the front side array lens and a first principal pointof the back side array lens is equal to the focal length of the backside array lens. A beam homogenizer of the third mode according to thepresent invention is equipped with a front side array lens of an opticalpath unshortened type, a back side array lens of an optical pathshortened type whose first principal point is positioned behind on abeam emission side, and a condensing lens, wherein the distance betweena second principal point of the front side array lens and the firstprincipal point of the back side array lens is equal to the focal lengthof the back side array lens.

In the beam homogenizer of the present invention that is categorizedinto the three modes, plural kinds of lenses, i.e., a combined arraylens or an array lens having curved surfaces on its opposite sides canbe used as the front side array lens and the back side array lens. Asthe combined array lens, plural kinds of arrays lenses, i.e., two ormore cylindrical lens arrays or fly-eye lenses can be used. Moreover, asthe array lens having curved surfaces on its opposite sides, any one ofplural kinds of lens arrays, i.e., a cylindrical lens array, a fly-eyelens, and a crossed cylindrical lens array each having curved surfaceson its front and back sides can be used.

As the curved surface of each lens, either a convex surface or a concavesurface can be used. At this time, it is preferable to use the same kindof lenses for the front side array lens and the back side array lens.For example, in the case of using a cylindrical lens array as ananterior lens of the front side array lens of an optical path shortenedtype, it is preferable to use cylindrical lens arrays of the same kindfor all of a posterior lens of the front side array lens, and anteriorand posterior lenses of the back side array lens. Moreover, as thecondensing lens, any one of a cylindrical lens, a toric lens, and acrossed cylindrical lens can be used.

Thus, many modes are applicable to the beam homogenizer of the presentinvention. The modes of the beam homogenizer of the present inventionare more specifically described hereinafter. As an array lens of anoptical path shortened type of the beam homogenizer of the presentinvention, a cylindrical lens array can be employed as described above,and in this case, it is preferable that, in the first mode, the frontside array lens of an optical path shortened type and the back sidearray lens of an optical path shortened type each includes twocylindrical lens arrays, a curved surface of an anterior cylindricallens array of the front side array lens is convex, a curved surface of aposterior cylindrical lens array of the front side array lens isconcave, a curved surface of an anterior cylindrical lens array of theback side array lens is concave, and a curved surface of a posteriorcylindrical lens array of the back side array lens is convex.

In the second mode, it is preferable that the front side array lens ofan optical path shortened type includes two cylindrical lens arrays, acurved surface of an anterior cylindrical lens array of the front sidearray lens is convex, a curved surface of a posterior cylindrical lensarray of the front side array lens is concave, and the back side arraylens of an optical path unshortened type is a cylindrical lens array.Moreover, in the third mode, it is preferable that the front side arraylens of an optical path unshortened type is a cylindrical lens array,the back side array lens of an optical path shortened type includes twocylindrical lens arrays, a curved surface of an anterior cylindricallens array of the back side array lens is concave, and a curved surfaceof a posterior cylindrical lens array of the back side array lens isconvex.

Moreover, as the array lens of an optical path shortened type of thebeam homogenizer of the present invention, a fly-eye lens array can beused as described above. In this case, in the first mode, it ispreferable that the front side array lens of an optical path shortenedtype and the back side array lens of an optical path shortened type eachincludes two fly-eye lenses, a curved surface of an anterior fly-eyelens of the front side array lens is convex, a curved surface of aposterior fly-eye lens of the front side array lens is concave, a curvedsurface of an anterior fly-eye lens of the back side array lens isconcave, and a curved surface of a posterior fly-eye lens of the backside array lens is convex.

In the second mode, it is preferable that the front side array lens ofan optical path shortened type includes two fly-eye lenses, a curvedsurface of an anterior fly-eye lens of the front side array lens isconvex, a curved surface of a posterior fly-eye lens is concave, and theback side array lens of an optical path unshortened type is a fly-eyelens. Further, in the third mode, it is preferable that the front sidearray lens of an optical path unshortened type is a fly-eye lens, theback side array lens of an optical path shortened type includes twofly-eye lenses, a curved surface of an anterior fly-eye lens of the backside array lens is concave, and a curved surface of a posterior fly-eyelens of the back side array lens is convex.

As the array lens of an optical path shortened type in the beamhomogenizer of the present invention, a cylindrical lens array havingcurved surfaces on its opposite sides can be used as described above. Inthis case, in the first mode, it is preferable that the front side arraylens of an optical path shortened type and the back side array lens ofan optical path shortened type each includes a cylindrical lens arrayhaving curved surfaces on its opposite sides, an anterior curved surfaceof the front side array lens is convex, a posterior curved surfacethereof is concave, an anterior curved surface of the back side arraylens is concave, and a posterior curved surface thereof is convex.

In the second mode, it is preferable that the front side array lens ofan optical path shortened type includes a cylindrical lens array havingcurved surfaces on its opposite sides, an anterior curved surface of thefront side array lens is convex, a posterior curved surface thereof isconcave, and the back side array lens of an optical path unshortenedtype is a cylindrical lens array having a curved surface on one side.Further, in the third mode, it is preferable that the front side arraylens of an optical path unshortened type is a cylindrical lens arrayhaving a curved surface on one side, the back side array lens of anoptical path shortened type is a cylindrical lens array having curvedsurfaces on its opposite sides, an anterior curved surface of the backside array lens is concave, and a posterior curved surface thereof isconvex.

A laser irradiation apparatus of the present invention is equipped withthe beam homogenizer of various kinds of modes described above and astage. The beam homogenizer homogenizes energy density distribution ofan irradiation beam in its short-side and long-side directions, and theirradiation beam with the energy density distribution homogenized in theboth directions is projected to an irradiation surface provided over thestage. Alternatively, the beam homogenizer homogenizes the energydensity distribution of anti irradiation beam in one of its short-sideand long-side directions and then homogenizes the energy densitydistribution of the irradiation beam in the other direction, and theirradiation beam with the energy density distribution homogenized in theboth directions is projected to an irradiation surface provided over thestage.

In the beam homogenizer of the present invention, by using the beamhomogenizer equipped with each kind of array lens of an optical pathshortened type including a lens system in which a convex lens and aconcave lens are combined or the like, the position of a principal pointof the lens system can be changed on purpose, whereby the distancebetween the lenses can be made short. Accordingly, the optical pathlength can be shortened. Therefore, it is possible to form a rectangularbeam spot, particularly a linear beam spot, with homogeneous energydistribution on an irradiation surface by a more compact optical system.In the present invention, by using the beam homogenizer of the presentinvention in a laser irradiation apparatus, the space occupied by theoptical system in the apparatus can be reduced, so that a more compactlaser irradiation apparatus with smaller foot print can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 schematically shows a beam homogenizer;

FIGS. 2A and 2B show a first principal point and a second principalpoint of a combined lens of a convex lens and a concave lens, andcomposite focal length of the combined lens;

FIGS. 3A and 3B schematically show a beam homogenizer of the presentinvention;

FIGS. 4A and 4B show a laser irradiation apparatus of the presentinvention;

FIGS. 5A and 5B show another mode of a laser irradiation apparatus ofthe present invention;

FIGS. 6A and 6B show a laser irradiation apparatus described in theRelated Art;

FIGS. 7A and 7B show a result of calculation on a laser irradiationapparatus of the present invention shown in FIGS. 4A and 4B by opticaldesign software;

FIGS. 8A and 8B show Embodiment 3;

FIGS. 9A and 9B show Embodiment 4;

FIGS. 10A and 10B show Embodiment 5; and

FIGS. 11A to 11C show structures of a fly-eye lens used in Embodiment 5.

DETAILED DESCRIPTION OF THE INVENTION Embodiment Mode

In Embodiment Mode of the present invention, a beam homogenizer using acylindrical lens array and a cylindrical lens is described first withreference to FIG. 1. A cylindrical lens array has a plurality ofcylindrical lenses arranged in a direction of their curvatures, and hasa role of dividing an incident beam into the same number of beams as thenumber of cylindrical lenses. As shown in FIG. 1, a cylindrical lensarray 102 with a focal length of f2 is arranged so as to have a distanceof f2 from a cylindrical lens array 101 with a focal length of f1. It isnoted that f1 is smaller than f2. Thus, incident light is divided intofive.

Here, the distance between the cylindrical lens arrays 101 and 102 meansthe distance from a second principal point of the cylindrical lens array101 to a first principal point of the cylindrical lens array 102. Atechnical definition of the first principal point and the secondprincipal point is later described. The light which has been dividedinto five by the cylindrical lens arrays 101 and 102 are combined by acylindrical lens 103 with a focal length of f3 on an irradiation surface104 arranged at a distance of f3 behind the cylindrical lens 103, atwhich the energy distribution is homogenized. In FIG. 1, the light isdivided into five; however, the homogeneity of the energy distributionincreases as the division number increases.

By arranging the optical system as shown in FIG. 1, a top surface ofeach cylindrical lens of the cylindrical lens array 101 is optically ina conjugate positional relation with the irradiation surface 104,whereby an image on the top surface is projected to the irradiationsurface 104. In other words, the irradiation surface 104 is irradiateduniformly by the respective cylindrical lenses of the cylindrical lensarray 101. It is natural that the sum of the energy distribution of thelaser beams from all the cylindrical lenses of the cylindrical lensarray is also homogeneous.

Therefore, even though the energy distribution is inhomogeneousdepending on a position or an orientation of light entering the wholesurface of the cylindrical lens array, it is possible to obtain a beamwith homogeneous energy distribution on an irradiation surface by usingthe beam homogenizer shown in FIG. 1. At this time, the length D of thebeam spot formed on the irradiation surface 104 is determined by thefollowing formula (1) where d is the width of the cylindrical lens ofthe cylindrical lens array.D=(f3/f2)d   formula (1)The distance between the cylindrical lens array 102 and the cylindricallens 103 does not depend on other parameters and can be freelydetermined.

As thus described, it is understood that a linear beam with homogeneousenergy distribution can be formed by using the optical system shown inFIG. 1 both in the long-side direction and the short-side direction ofthe linear beam. At that time, after the linear beam is homogenized inthe short-side direction, the linear beam may be homogenized in thelong-side direction, or the order may be opposite. If the linear beam ishomogenized in the long-side direction first, a stripe pattern parallelto the short-side direction may appear on the irradiation surface;therefore, it is preferable that the energy distribution thereof in theshort-side direction is homogenized first and then the energydistribution thereof in the long-side direction is homogenized.

In consideration of the above, for example, when designing a beamhomogenizer for the long-side direction in order to form a linear beamwhich is very long in the long-side direction, f3 in the formula (1),which is the focal length of the cylindrical lens 406, is made small.However, this means that the beam is extended suddenly in a shortdistance, which makes it difficult to homogenize the energy distributionin the long-side direction sufficiently. Therefore, it is necessary tosecure distance for extending the beam spot in the long-side direction(distance from a second plane of the cylindrical lens 406 to theirradiation surface). Accordingly, the focal length of the condensinglens in the short-side direction (cylindrical lens 407) needs to beextended. The width of the beam spot in the short-side direction isdetermined by the formula (1).

According to the formula (1), when f3 (the focal length of thecylindrical lens 407) is increased, it is necessary to increase f2 (thecomposite focal length of the cylindrical lens arrays 404 a and 404 b).As thus described, it is understood that size reduction of the opticalsystem for homogenizing the energy distribution in the short-sidedirection is effective to reduce the size of the entire optical system.Therefore, when a linear beam with longer length in the long-sidedirection is to be formed, it is necessary to decrease f2 of the opticalsystem for homogenizing the energy distribution in the short-sidedirection as derived from the formula (1).

In FIGS. 4A and 4B, decreasing f2 (the composite focal length of thecylindrical lens arrays 404 a and 404 b) in the optical system meansnothing less than decreasing the distance between a second principalpoint of a lens system including cylindrical lens arrays 403 a and 403 band a first principal point of a lens system including the cylindricallens arrays 404 a and 404 b. Now, the positions of the principal pointsof the lenses are considered. If light enters one lens, which is one ofmass-produced lenses with a small lens thickness, for example, a lensappearing in catalogs of laser manufactures, a first principal point anda second principal point of the lens both exist within a lens or on alens surface.

The first principal point and the second principal point of the lens aredefined as follows. The lens has two principal points as well as a focalpoint, which are a first principal point and a second principal point,and the first principal point is as follows. If it is assumed that whena ray having passed through a front side (left side) focal point (i.e.,a ray traveling in parallel with an optical axis after passing through alens) passes through the lens, the ray is refracted once at a virtualline instead of refracting twice at top and rear surfaces of the lens,the virtual plane can be defined in fact and is referred to as a firstprincipal plane, and an intersection of the plane and the optical axisis referred to as the first principal point.

In other words, a virtual plane in the case that when the ray enteringthe lens in parallel with the optical axis from the right side passesthrough the lens, the ray is refracted once at a virtual line instead ofrefracting twice at the top and rear surfaces of the substrate isreferred to as a first principal plane, and an intersection of the planeand the optical axis is referred to as a first principal point. On thecontrary, a virtual plane in the case that when the ray having passedthrough a back side (right side) focal point actually passes through thelens, the ray is refracted once at the virtual plane instead ofrefracting twice at the top and rear surfaces of the substrate isreferred to as a second principal plane, and an intersection of theplane and the optical axis is referred to as a second principal point.

In contrast, in the case of using a lens system in which lenses arecombined, the position where the principal point is formed variessignificantly depending on the lens curvature or the distance betweenthe lenses, and is sometimes formed outside the lens. For example, asshown in FIG. 2A, if a first lens 201 is a convex lens and a second lens202 is a concave lens, the principal point is formed ahead of the firstlens 201. On the other hand, if a first lens 203 is a concave lens and asecond lens 204 is a convex lens, the principal point is formed behindthe second lens.

Therefore, as compared with the case of using one lens, the position ofthe second principal point moves ahead of the lens system in the case ofusing the lens system shown in FIG. 2A, while the position of the firstprincipal point moves behind the lens system in the case of using thelens system shown in FIG. 2B. As thus described, it has been confirmedthat the position of the principal point can be controlled by using thelens system including a plurality of lenses. The above description ismade on the items for helping understanding of the present invention.Now, Embodiment Mode of the present invention will be described indetail based on the above description.

Embodiment Mode of the present invention will hereinafter describe anembodiment mode of a beam homogenizer in detail and also brieflydescribe an embodiment mode of a laser irradiation apparatus. Thepresent invention is not limited by the description of the embodimentmodes and is specified by the scope of claims. Moreover, it is easilyunderstood by those skilled in the art that the mode and detail can bevariously changed without departing from the spirit and scope of thepresent invention. In the structure of the present invention hereinaftershown, the reference numeral indicating the same thing is used in commonin different figures.

First, an embodiment mode of a beam homogenizer will be described withreference to FIGS. 3A and 3B. FIG. 3A shows a beam homogenizer with thesame structure as that in FIG. 1, in which a second principal point X ofthe cylindrical lens array 101 is formed at a position A and a firstprincipal point Y of the cylindrical lens array 102 is formed at aposition B, and the distance between the second principal point X andthe first principal point Y becomes f2. Meanwhile, FIG. 3B shows a beamhomogenizer including the lens system shown in FIGS. 2A and 2B.

A lens system including a cylindrical lens array 301 and a cylindricallens array 302 is referred to as a first lens system (corresponding to afront side array lens of an optical path shortened type), and a lenssystem including a cylindrical lens array 303 and a cylindrical lensarray 304 is referred to as a second lens system (corresponding to aback side array lens of an optical path shortened type). The lenscurvature and the distance between the lenses (the distance between asecond principal point of the cylindrical lens array 301 and a firstprincipal point of the cylindrical lens array 302) in the first lenssystem are set so that the composite focal length becomes f1. Similarlyto the lens system shown in FIG. 2A, a laser beam enters a concave lensafter a convex lens.

Thus, a second principal point C of the first lens system is formedahead of the cylindrical lens array 301. Therefore, the first lenssystem can be moved back in order to match the second principal point Xof the cylindrical lens array 101 to the second principal point C of thefirst lens system. That is to say, it is possible to shorten the opticalpath length by the distance of the movement of the second principalpoint of the first lens system.

Moreover, the lens curvature and the distance between the lenses (thedistance between the cylindrical lens array 303 and the cylindrical lensarray 304) in the second lens system are set so that the composite focallength becomes f2. Similarly to the lens system shown in FIG. 2B, alaser beam enters a convex lens after a concave lens. Thus, a firstprincipal point D of the second lens system is formed behind thecylindrical lens array 304. Therefore, the second lens system can bemoved ahead in order to match the second principal point Y of thecylindrical lens array 102 to the first principal point D of the secondlens system.

As described with reference to FIG. 1, since the distance between thecylindrical lens array 102 and the cylindrical lens 103 is notparticularly limited, the cylindrical lens 305 can be made closer to thesecond lens system by the distance of the movement of the second lenssystem. That is to say, if the position of the cylindrical lens 305 isfixed, the first lens system and the second lens system can be moved bythe distance of the movement of the first principal point of the secondlens system, and accordingly the optical path length can be shortened bythe distance of the movement of the first principal point of the secondlens system.

In the beam homogenizer shown in FIG. 3B, combined cylindrical lensarrays are used as the front side array lens and the back side arraylens. This is the preferred mode of the present invention; however, atleast one combined cylindrical lens array is necessary in the beamhomogenizer of the present invention, and for example the front sidearray lens may be replaced by one cylindrical lens array, i.e., an arraylens of an optical path unshortened type, as shown in FIGS. 5A and 5Bwhich illustrate one mode of a laser irradiation apparatus.

Although not shown, the back side array lens instead of the front sidearray lens may be replaced by one cylindrical lens array, i.e., an arraylens of an optical path unshortened type. Even in this case, since onecombined cylindrical lens array is employed, it is possible to shortenthe optical path length and reduce the size of the optical system.However, the optical path length can be shortened more and the opticalsystem can be reduced in size more by using two combined cylindricallens arrays like in the beam homogenizer shown in FIG. 3B.

Although the beam homogenizer of the present invention is used tohomogenize the beam in the short-side direction in this embodiment mode,the beam homogenizer of the present invention can be used to homogenizethe beam in the long-side direction and also can be used to homogenizethe beam in both of the short-side direction and the long-sidedirection. In the beam homogenizer shown in FIG. 3B, the beams dividedby both of the front side and back side combined cylindrical lens arraysare combined by the cylindrical lens 305 provided just behind them,whereby the energy distribution of the beam is homogenized.

As for an embodiment mode of a laser irradiation apparatus, two kinds ofmodes are described briefly with reference to FIGS. 4A and 4B, and FIGS.5A and 5B. FIGS. 4A and 4B show a laser irradiation apparatus using thebeam homogenizer shown in FIG. 3B. That is to say, in this laserirradiation apparatus, two combined cylindrical lens arrays (the frontside combined cylindrical lens arrays 403 a and 403 b and the back sidecombined cylindrical lens arrays 404 a and 404 b) are used and thecombined cylindrical lens arrays 403 a and 403 b, and 404 a and 404 bcorrespond to the front side array lens of an optical path shortenedtype and the back side array lens of an optical path shortened type inthe beam homogenizer of the present invention. This corresponds to thefirst mode described above.

Thus, the beam is divided in the short-side direction, and the dividedbeams are condensed (combined) by the cylindrical lens 407, so that theenergy distribution in the short-side direction is homogenized.Moreover, cylindrical lens arrays 405 a and 405 b for dividing the beamin the long-side direction are arranged ahead of the cylindrical lens407 in the traveling direction of the beam, and the beams divided in thelong-side direction by these cylindrical lens arrays 405 a and 405 b arecombined by the cylindrical lens 406, whereby the energy distributionthereof is homogenized.

In the laser irradiation apparatus shown in FIGS. 4A and 4B, thecylindrical lens 407 corresponds to a condensing lens of the beamhomogenizer of the present invention. Although the cylindrical lens 407is arranged behind the cylindrical lens 406 and the cylindrical lensarrays 405 a and 405 b that form the beam homogenizer for the long-sidedirection in FIGS. 4A and 4B, the cylindrical lens 407 may be arrangedahead of the cylindrical lens arrays 405 a and 405 b differently fromthe arrangement shown in the figure.

A laser irradiation apparatus shown in FIGS. 5A and 5B is of a differentmode from the laser irradiation apparatus shown in FIGS. 4A and 4B, anda beam homogenizer used in FIGS. 5A and 5B has a cylindrical lens array503, which is an incomposite array lens of an optical path unshortenedtype, instead of the front side combined cylindrical lens array in thebeam homogenizer shown in FIG. 3B. However, even in this case, since onecombined cylindrical lens is used, it is possible to shorten the opticalpath length and reduce the size of the optical system. It is to be notedthat the beam homogenizer used in the laser irradiation apparatus inFIGS. 5A and 5B corresponds to the third mode.

However, in this case, the optical path length is not shortened thatmuch as compared with the case of using combined cylindrical lens arrayswhich are array lenses of an optical path shortened type as the frontside and back side array lenses like in the beam homogenizer shown inFIGS. 4A and 4B, because the array lens is used on only one side. Inother words, if combined cylindrical lens arrays which are array lensesof an optical path shortened type are used as the front side and backside array lenses like in the beam homogenizer shown in FIG. 4A and 4B,the optical path length can be shortened as compared with the case ofusing a combined cylindrical lens array which is an array lens of anoptical path shortened type as only the array lens on one side shown inFIGS. 5A and 5B. The two modes of the laser irradiation apparatus willbe specifically described in the following embodiment.

Embodiment 1

Embodiments of the present invention are hereinafter described withreference to the drawings. However, the present invention is not limitedby the description of Embodiments but specified by the scope of claims.It is easily understood by those skilled in the art that the modes anddetails can be variously changed without departing from the spirit andscope of the present invention. In the structures of the presentinvention described hereinafter, the reference numeral indicating thesame part is used in common in different drawings.

FIGS. 4A and 4B show an optical system employed in Embodiment 1.Embodiment 1 is described with reference to a side view of FIG. 4B. Thisside view includes in its paper a short-side direction of a linear beamspot formed by the optical system. A laser beam emitted from a XeClexcimer laser 401 travels in a direction indicated by arrows in FIGS. 4Aand 4B. First, the laser beam is expanded by spherical lenses 402 a and402 b. This structure is not necessary if the beam spot emitted from thelaser oscillator 401 is sufficiently large. The laser beam emitted fromthe laser oscillator is divided in the short-side direction bycylindrical lens arrays which will be described below.

As described above, the long-side direction and the short-side directionare the same as a direction where the width of a linear beam spot on anirradiation surface 409 is longer and a direction where the widththereof is shorter, respectively. A surface of a lens where light entersis referred to as a first surface while a surface of a lens where lightis emitted is referred to as a second surface. The sign of radius ofcurvature is positive when the center of curvature is on a side wherelight is emitted with respect to the lens surface and negative when thecenter of curvature is on a side where light enters with respect to thelens surface. Moreover, the lenses used in this embodiment aremanufactured with synthetic quartz having high transmittance and laserresistance against a XeCl excimer laser with a wavelength of 308 nm.

The cylindrical lens array 403 a has eleven cylindrical lenses arrangedin a direction of the curvature, each of which has a first surface witha radius of curvature of 146.8 mm, a second surface which is plane, athickness of 5 mm, and a width of 4 mm in the short-side direction. Thecylindrical lens array 403 b has eleven cylindrical lenses arranged in adirection of the curvature, each of which has a first surface which isplane, a second surface with a radius of curvature of 160 mm, athickness of 5 mm, and a width of 4 mm in the short-side direction. Thedistance between a second surface of the cylindrical lens array 403 aand a first surface of the cylindrical lens array 403 b is 85 mm, thecomposite focal length of the cylindrical lens array 403 a and thecylindrical lens array 403 b is 837.5 mm, and a second principal pointof a lens system including the two lenses is formed at 162.4 mm ahead ofthe first plane of the cylindrical lens array 403 a.

The cylindrical lens array 404 a has eleven cylindrical lenses arrangedin a direction of the curvature, each of which has a first surface witha radius of curvature of −262.4 mm, a second surface which is plane, athickness of 5 mm, and a width of 4 mm in the short-side direction. Thecylindrical lens array 404 b has eleven cylindrical lenses arranged in adirection of the curvature, each of which has a first surface which isplane, a second surface with a radius of curvature of −200 mm, athickness of 5 mm, and a width of 4 mm in the short-side direction. Whenthe cylindrical lens arrays 404 a and 404 b are arranged so that thedistance between a second surface of the cylindrical lens array 404 aand a first surface of the cylindrical lens array 404 b is 60 mm, thecomposite focal length of the cylindrical lens arrays 404 a and 404 b is1139.8 mm, and a first principal point of a lens system including thetwo lenses is formed at about 118 mm behind a second surface of thecylindrical lens array 404 b.

This composite focal length (f) and the distance (z) from a secondprincipal point of the second lens to a second principal point of thecombined system can be calculated from known formulas, and for example,the composite focal length of the combined lens including two lenses canbe obtained from the following formula (2) and the distance can beobtained from the following formula (3), where L is a distance of theprincipal point of the two lenses.1/f=1/f1+1/f2−L/f1f2   formula (2)z=−f2L(f1+f2−L)   formula (3)

The cylindrical lens array 404 a is arranged at 694.4 mm behind a secondsurface of the cylindrical lens array 403 b so that the distance betweena second principal point of the front side combined cylindrical lensarray including the cylindrical lens array 403 a and the cylindricallens array 403 b and a first principal point of the back side combinedcylindrical lens array including the cylindrical lens array 404 a andthe cylindrical lens array 404 b is 1139.8 mm, which is equal to thecomposite focal length of the cylindrical lens array 404 a and thecylindrical lens array 404 b.

The spots divided by the cylindrical lens arrays 403 a, 403 b, 404 a,and 404 b are condensed by the cylindrical lens 407 which has a firstsurface with a radius of curvature of 486 mm, a second surface that isplane, and a thickness of 20 mm and which is provided at 1815 mm behindthe second surface of the cylindrical lens array 404 b. Thus, a plane410 with homogeneous energy distribution which has a length of 3.5 mm inthe short-side direction is formed at about 1000 mm behind a secondsurface of the cylindrical lens 407.

As thus described, it is possible to shorten the optical path length byabout 283.6 mm as compared with the case of replacing the convexcylindrical lens array 403 a and the concave cylindrical lens array 403b with one convex cylindrical lens array having a first surface that iscurved, a second surface that is plane, a thickness of 5 mm, and thesame focal length as the composite focal length of the convexcylindrical lens array 403 a and the concave cylindrical lens array 403b and replacing the concave cylindrical lens array 404 a and the convexcylindrical lens array 404 b with a cylindrical lens array having afirst surface that is plane, a second surface that is curved, athickness of 5 mm, and the same focal length as the composite focallength of the concave cylindrical lens array 404 a and the convexcylindrical lens array 404 b. Thus, a combination of the cylindricallens arrays 403 a, 403 b, 404 a, and 404 b and the cylindrical lens 407serves as a beam homogenizer for the short-side direction, and thiscorresponds to the beam homogenizer of the present invention.

The plane 410 with the homogeneous energy distribution formed by thebeam homogenizer is projected to an irradiation surface provided at 220mm behind a second surface of a doublet cylindrical lens 408 b providedat 1250 mm behind the plane with the homogenous energy distribution, bythe doublet cylindrical lens 408 b. In other words, the homogeneousplane 410 and the irradiation surface 409 are in a conjugate relationwith respect to the doublet cylindrical lens 408. This homogenizes theenergy distribution of the linear beam spot in the short-side directionand determines the length thereof in the short-side direction. Thedoublet cylindrical lens 408 includes a cylindrical lens 408 a and acylindrical lens 408 b.

The cylindrical lens 408 a has a first surface with a radius ofcurvature of 125 mm, a second surface with a radius of curvature of 77mm, and a thickness of 10 mm. The cylindrical lens 408 b has a firstsurface with a radius of curvature of 97 mm, a second surface with aradius of curvature of −200 mm, and a thickness of 20 mm. The distancebetween the second surface of the cylindrical lens 408 a and the firstsurface of the cylindrical lens 408 b is 5.5 mm. When the homogeneity ofthe beam spot on the irradiation surface is not required that much orwhen an F-number (F=focal length of the lens/entrance pupil diameter) ofthe doublet cylindrical lens is very large, a singlet cylindrical lensmay be used.

Next, a plan view of FIG. 4A is described. The plan view includes in itspaper a long-side direction of the linear beam spot formed by theoptical system. The spot of the laser beam emitted from the laseroscillator 401 is divided in the long-side direction by the cylindricallens arrays 405 a and 405 b. The cylindrical lens array 405 a has twelvecylindrical lenses arranged in a direction of the curvature, each ofwhich has a first surface with a radius of curvature of 40 mm, a secondsurface that is plane, a thickness of 3 mm, and a width of 9 mm in thelong-side direction. The cylindrical lens array 405 b has twelvecylindrical lenses arranged in a direction of the curvature, each ofwhich has a first surface that is plane, a second surface with a radiusof curvature of −55 mm, a thickness of 3 mm, and a width of 9 mm in thelong-side direction.

The cylindrical lens arrays 405 a and 405 b are arranged so that thedistance between a second principal point of the cylindrical lens array405 a and a first principal point of the cylindrical lens array 405 b is113.3 mm, which is equal to the focal length of the cylindrical lensarray 405 b. The spots divided by the cylindrical lens arrays 405 a and405 b are superposed on the irradiation surface 409 by the cylindricallens 406 which has a first surface that is plane and a second surfacewith a radius of curvature of −2140 mm and which is provided at 82 mmbehind the second surface of the cylindrical lens array 405 b. Thishomogenizes the energy distribution of the linear beam spot in thelong-side direction and determines the length thereof in the long-sidedirection.

It is possible to decrease a portion where the energy attenuates atopposite ends of the linear beam spot in the long-side direction byusing the cylindrical lens 406. However, the focal length of the lensmay be extremely long because of the structure of the apparatus. In sucha case, the lens does not always have to be used because theadvantageous effect of this lens decreases. Thus, a combination of thecylindrical lens arrays 405 a and 405 b and the cylindrical lens 406corresponds to a beam homogenizer in the long-side direction but doesnot correspond to the beam homogenizer of the present invention.

As thus described, a linear beam spot with homogeneous energydistribution having a length of 700 μm in the short-side direction and300 mm in the long-side direction can be formed on the irradiationsurface 409 by using the optical system shown in FIGS. 4A and 4B. FIGS.7A and 7B show a result of ray trace calculation of the optical systemshown in FIGS. 4A and 4B obtained by using optical design software. Thevertical axis shows the intensity of the obtained beam spot while thehorizontal axis shows the lengths of a short side and a long side of thebeam spot.

The laser oscillator used in combination with the beam homogenizer ofthe present invention preferably has high output power and a wavelengthrange which is sufficiently absorbed in a semiconductor film. In thecase of using a silicon film as a semiconductor film, the wavelength ofthe laser beam emitted from the laser oscillator is preferably 600 nm orshorter. As the laser oscillator emitting such a laser beam, forexample, an excimer laser, a YAG laser (harmonic), or a glass laser(harmonic) is given.

Moreover, as a laser oscillator emitting a laser beam with anappropriate wavelength for crystallizing a silicon film, for example, aYVO₄ laser (harmonic), a YLF laser (harmonic), an Ar laser, a GdVO₄laser (harmonic), or a Ti:sapphire laser (harmonic) is given. Theoptical system of the present invention may be used in the air or undera nitrogen or argon atmosphere in order to suppress breakdown or damageof a lens surface due to a laser beam with high energy.

Embodiment 2

This embodiment shows an example of a different optical system from theoptical system described above, with reference to FIGS. 5A and 5B. InFIGS. 5A and 5B, a laser beam travels along the same optical path asthat in the optical system shown in FIGS. 4A and 4B described inEmbodiment 1 except the optical path in a cylindrical lens array thatforms a homogenizer in a short-side direction. A beam homogenizer shownin this embodiment is described with reference to a side view of FIG 5B.It is to be noted that the lenses shown in this embodiment are made ofsynthetic quartz having high transmittance and laser resistance againsta XeCl excimer laser.

The laser beam emitted from the laser oscillator is divided by acylindrical lens array 503 hereinafter described in a short-sidedirection. The cylindrical lens array 503 has eleven cylindrical lensesarranged in a direction of the curvature, each of which has a firstsurface with a radius of curvature of 412.8 mm, a second surface that isplane, a thickness of 5 mm, and a width of 4 mm in the short-sidedirection. A second principal point of the cylindrical lens array 503 isformed inside the lens at about 3.6 mm from a second surface of thecylindrical lens.

The cylindrical lens array 404 a has eleven cylindrical lenses arrangedin a direction of the curvature, each of which has a first surface witha radius of curvature of −262.4 mm, a second surface that is plane, athickness of 5 mm, and a width of 4 mm in the short-side direction. Thecylindrical lens array 404 b has eleven cylindrical lenses arranged in adirection of the curvature, each of which has a first surface that isplane, a second surface with a radius of curvature of −200 mm, athickness of 5 mm, and a width of 4 mm in the short-side direction. Thedistance between a second surface of the cylindrical lens array 404 aand a first surface of the cylindrical lens array 404 b is 60 mm, thecomposite focal length of the cylindrical lens array 404 a and thecylindrical lens array 404 b is 1139.8 mm, and the first principal pointof a lens system including the two lenses is formed at about 118 mmbehind the second surface of the cylindrical lens array 404 b.

The cylindrical lens array 404 a is arranged at 948.2 mm behind thesecond surface of the cylindrical lens array 503 so that the distancebetween the second principal point of the cylindrical lens array 503 andthe first principal point of the lens system including the cylindricallens array 404 a and the cylindrical lens array 404 b is 1139.8 mm,which is equal to the composite focal length of the cylindrical lensarray 404 a and the cylindrical lens array 404 b.

The spots divided by the cylindrical lens arrays 503, 404 a, and 404 bare condensed by the cylindrical lens 407 which has a first surface witha radius of curvature of 486 mm, a second surface that is plane, and athickness of 20 mm and which is provided at 1815 mm behind the secondsurface of the cylindrical lens array 404 b, thereby forming a planewith homogeneous energy distribution having a length of 3.6 mm in theshort-side direction at 1000 mm behind the second surface of thecylindrical lens 405. It is possible to shorten the optical path lengthby about 119.6 mm as compared with the case of replacing the concavecylindrical lens array 404 a and the convex cylindrical lens array 404 bwith one convex cylindrical lens array having the same focal length asthe composite focal length of the cylindrical lens arrays 404 a and 404b.

Embodiment 3

This embodiment will show an example of a different optical system fromthe optical system shown above, with reference to FIGS. 8A and 8B.Similarly to another embodiment, the lenses shown in this embodiment aremade of synthetic quartz having high transmittance and laser resistanceagainst a XeCl excimer laser; however, the material of the lens can beselected appropriately in accordance with a laser and a wavelength rangeto be used. FIG. 8A shows only an optical system for homogenizing theenergy density distribution of the beam in the short-side direction. Theoptical system for homogenizing the energy density distribution in thelong-side direction is used similarly to another embodiment.

The cylindrical lens arrays 403 a and 403 b used here correspond to thefront side array lens of an optical path shortened type in the beamhomogenizer of the present invention, and FIG. 8B shows an opticalsystem in which a cylindrical lens array 801 with both of its firstsurface and second surface having different curvature is used instead ofthe cylindrical lens arrays 403 a and 403 b. The cylindrical lens array801 has a first surface with a radius of curvature of 47.8 mm, a secondsurface with a radius of curvature of 50.5 mm, a thickness of 10 mm, anda focal length of 832.8 mm. A second principal point of the cylindricallens array 801 is formed at 56.9 mm from a second surface toward a firstsurface, i.e., 46.9 mm ahead of the first surface.

The cylindrical lens array 801 is arranged so that the distance betweenthe second principal point of the cylindrical lens array 801 and thefirst principal point of the lens system (hereinafter referred to as asecond lens system) including the cylindrical lens array 404 a and thecylindrical lens array 404 b is 1139.8 mm, which is equal to thecomposite focal length of the second lens system. In other words, thedistance between the second surface of the cylindrical lens array 801and the first surface of the cylindrical lens array 404 a is 894.9 mm.

It is possible to shorten the optical path length by 48.5 mm by usingthe optical system shown in FIGS. 8A and 8B as compared with the case ofusing, instead of the cylindrical lens array 801 having curved surfaceson its opposite sides, one planoconvex cylindrical lens array (with afirst surface curved and a thickness of 5 mm) with the same focal lengthas the cylindrical lens array 801. Although this embodiment has shown anexample of homogenizing the energy density distribution in theshort-side direction, the energy density distribution in the long-sidedirection or the energy density distribution in the long-side andshort-side directions may be homogenized.

As thus described, this embodiment provides an advantageous effect thatthe optical path length can be shortened as compared with the case ofusing one planoconvex lens when the cylindrical lens array 403 a and thecylindrical lens array 403 b are replaced by the cylindrical lens array801 with opposite surfaces curved. Moreover, a similar advantageouseffect can be obtained even when the cylindrical lens arrays 404 a and404 b are replaced by the cylindrical lens array 801 with oppositesurfaces curved.

Further, the cylindrical lens arrays 403 a and 403 b may be replaced bythe cylindrical lens array 801 with opposite surfaces curved, andmoreover the cylindrical lens arrays 404 a and 404 b may be replaced byone cylindrical lens array with opposite surfaces curved. In such acase, an effect of shortening an optical path can be obtained byreplacing the front side and back side lens arrays. That is to say, aneffect of shortening an optical path in the case of using a front sidearray lens of an optical path shortened type and a back side array lensof an optical path shortened type can be obtained.

Embodiment 4

This embodiment will show an example in which the width of a beam isadjusted by using the optical system described in another embodimentsuch as Embodiment 1. The lenses shown in this embodiment are made ofsynthetic quartz having high transmittance and laser resistance againsta XeCl excimer laser; however, the material of the lenses can beappropriately selected in accordance with a laser and a wavelength rangeto be used. FIG. 9A shows only an optical system related to theshort-side direction of the beam among the optical system shown FIG. 4B.The optical system related to the long-side direction of the beam may besimilar to that in another embodiment. A point 410 at which the energydistribution is homogeneous is the focal point of the cylindrical lens407.

The beam width D at this point is expressed with the following formula(4) where d is the width of each lens in the short-side direction of thecylindrical lens arrays 403 a, 403 b, 404 a, and 404 b; f404 is thecomposite focal length of the cylindrical lens arrays 404 a and 404 b;and f407 is the focal length of the cylindrical lens 407.D=(f407/f404)×d   formula (4)By assigning f404=1139.8 mm, f407=1000.8 mm, and d=4 mm, D=3.5 mm isobtained from the above formula (4). When this beam is projected using alens with a magnification of 5, a beam of 700 μm wide can be formed.

Here, as shown in FIG. 9B, the width of the beam formed at the point 410at which the energy density distribution is homogeneous can be changedby changing the distance between the second surface of the cylindricallens array 404 a and the first surface of the cylindrical lens array 404b. However, if the distance between these lenses is changed, the firstprincipal point of a lens system including the cylindrical lens arrays404 a and 404 b (second lens system) moves. Therefore, it is necessaryto move another lens so that the distance from the second principalpoint of a lens system including the cylindrical lens arrays 403 a and403 b (hereinafter referred to as a first lens system) to the firstprincipal point of the second lens system is equal to f404.

For example, in order to change the width of the beam formed by theoptical system shown in FIG. 9A, the cylindrical lens array 404 a ismoved by +15.5 mm in a z-direction in FIGS. 9A and 9B, so that thedistance between the second surface of the cylindrical lens array 404 aand the first surface of the cylindrical lens array 404 b is shortenedfrom 60 mm to 44.5 mm. By this operation, the composite focal length ofthe second lens system is changed from 1139.8 mm to 1238.2 mm. A z-axisis parallel to the optical axis, and a direction along which the lighttravels is denoted as positive (+).

With the movement of the cylindrical lens array 404 a, the firstprincipal point of the second lens system moves. The first lens systemis wholly moved by −113.6 mm in the z-axis direction of FIG. 9A and 9Bso that the distance between the second principal point of the firstlens system and the first principal point of the second lens system isequal to the composite focal length of the second lens system. By movingthe cylindrical lens array 404 a, the focal length f404 becomes 1238.2mm.

Then, as can be confirmed by assigning in the formula (1), a beam with awidth of 3.23 mm is formed at the point 410 where the energy densitydistribution is homogeneous. Moreover, a beam with a length of 646 μmcan be formed by projecting the beam onto another plane with a lenshaving a magnification of 5. Thus, when the beam homogenizer of thepresent invention is used, the width of the beam to be formed can bechanged by moving the lens position. Although this embodiment has shownthe optical system used in Embodiment 1, this embodiment can be combinedwith another embodiment as well.

Embodiment 5

This embodiment will show an example of using a fly-eye lens (alsoreferred to as an integrator) as a lens for homogenizing a beam andshortening optical path length. The fly-eye lens is an aggregation oflenses with a compound eye structure, and is formed by arranging a groupof one kind of lens elements (like a rod lens) each having oppositesurfaces curved or formed by arranging a group of lens elements in sucha way that a group of lens elements on an incidence side and lenses onan emission side face each other. The fly-eye lens has a function toproject light having entered the lens surface on an incidence side ontothe irradiation surface by passing though the emission side of the lens.

When the respective projected light is summed on the irradiationsurface, the intensity distribution on the irradiation surface ishomogenized. The shape of the beam spot on the irradiation surfacereflects the shape of the lens element. This embodiment shows an exampleof using a fly-eye lens whose lenses have square shapes; however, afly-eye lens whose lenses have rectangular or triangle shapes may alsobe used similarly.

Similarly to another embodiment, the lenses shown in this embodiment aremade of synthetic quartz having high transmittance and laser resistanceagainst a XeCl excimer laser; however, the material of the lens can beappropriately selected in accordance with a laser or a wavelength rangeto be used. FIGS. 10A and 10B show an optical system including fly-eyelenses as a homogenizer acting in a first direction of the beam and asecond direction perpendicular to the first direction. It is to be notedthat FIG. 10A is a plan view while FIG. 10B is a side view. Since theshape of each lens which forms the fly-eye lens is square in thisembodiment, the beam is homogenized in the same way in the firstdirection and the second direction.

A laser beam emitted from a XeCl excimer laser oscillator 1001 travelsalong a direction indicated by an arrow in FIGS. 10A and 10B. First, thelaser beam is expanded by spherical lenses 1002 a and 1002 b. Thisstructure is not necessary if the beam diameter of the laser beamemitted from the laser oscillator 1001 is large. A spot of the laserbeam emitted from the laser oscillator 1001 is divided in the firstdirection and the second direction by a fly-eye lens hereinafterdescribed.

The fly-eye lens 1003 a has 11×11 spherical lenses arranged in a firstdirection and a second direction as shown in FIG. 11B, each of which hasa first surface with a radius of curvature of 146.8 mm, a second surfacethat is plane, a thickness of 5 mm, a length of 4 mm in the first andsecond directions as shown in FIG. 11A. The fly-eye lens 1003 b has11×11 spherical lenses arranged in the first direction and the seconddirection as shown in FIG. 11B, each of which has a first surface thatis plane, a second surface with a radius of curvature of 160 mm, athickness of 5 mm, and a length of 4 mm in the first and seconddirections.

The distance between a second surface of the fly-eye lens 1003 a and afirst surface of the fly-eye lens 1003 b is 85 mm, and the compositefocal length of the fly-eye lens 1003 a and the fly-eye lens 1003 b is837.5 mm. A second principal point of a lens system including these twolenses is formed at 162.4 mm ahead of the first surface of the fly-eyelens 1003 a. The fly-eye lens 1004 a has 11×11 spherical lenses arrangedin the first direction and the second direction, each of which has afirst surface with a radius of curvature of −262.4 mm, a second surfacethat is plane, a thickness of 5 mm, and a length of 4 mm in the firstand second directions. The fly-eye lens 1004 b has 11×11 sphericallenses arranged in the first direction and the second direction, each ofwhich has a first surface that is plane, a second surface with a radiusof curvature of −200 mm, a thickness of 5 mm, and a length of 4 mm inthe first and second directions.

When the fly-eye lenses 1004 a and 1004 b are arranged so that thedistance between a second surface of the fly-eye lens 1004 a and a firstsurface of the fly-eye lens 1004 b is 60 mm, the composite focal lengthof the fly-eye lenses 1004 a and 1004 b becomes 139.8 mm and a firstprincipal point of a lens system including these two lenses is formed atabout 118 mm behind the second surface of the fly-eye lens 1004 b. It isto be noted that in a combined lens system including two lenses, thefocal length f and the distance z from a second principal point of asecond lens to a second principal point of the combined lens system canbe calculated from the formulas (2) and (3).

The fly-eye lens 1004 a is arranged at 694.4 mm behind the secondsurface of the fly-eye lens 1003 b so that the distance between a secondprincipal point of a front side combined lens including the fly-eye lens1003 a and the fly-eye lens 1003 b and a first principal point of a backside combined lens including the fly-eye lens 1004 a and the fly-eyelens 1004 b is 1139.8 mm which is equal to the composite focal length ofthe fly-eye lens 1004 a and the fly-eye lens 1004 b. The spots dividedby the fly-eye lenses 1003 a, 1003 b, 1004 a, and 1004 b are condensedby a spherical lens 1005 which has a first surface with a radius ofcurvature of 486 mm, a second surface that is plane, and a thickness of20 mm and which is provided at 1815 mm behind the second surface of thefly-eye lens 1004 b, thereby forming a square plane 1008 with a lengthof 3.5 mm on each side at which the energy distribution is homogeneousat about 1000 mm behind a second surface of the spherical lens 1005.

It is possible to shorten the optical path length by about 283.6 mm ascompared with the case of replacing the fly-eye lenses 1003 a and 1003 bwith one fly-eye lens having a first surface which is curved, a secondsurface which is plane, a thickness of 5 mm, and the same focal lengthas the composite focal length of the fly-eye lenses 1003 a and 1003 band replacing the fly-eye lenses 1004 a and 1004 b with one fly-eye lenshaving a first surface which is plane, a second surface which is curved,a thickness of 5 mm, and the same focal length as the composite focallength of the fly-eye lenses 1004 a and 1004 b. A combination of thefly-eye lenses 1003 a, 1003 b, 1004 a, and 1004 b and the spherical lens1005 corresponds to the beam homogenizer of the present invention.

A plane 1008 formed by the beam homogenizer of the present invention atwhich the energy distribution is homogeneous is projected to anirradiation surface 1007 provided at 220 mm behind a second surface of aspherical lens 1006 b by a doublet lens 1006 arranged at 1250 mm behindthe plane 1008 at which the energy distribution is homogeneous. In otherwords, the plane 1008 at which the energy distribution is homogeneousand the irradiation surface 1007 are in a conjugate relation withrespect to the doublet lens 1006. This homogenizes the energydistribution of the square beam spot in the first and second directions,and determines the length thereof in the first and second directions.The doublet lens 1006 includes a spherical lens 1006 a and the sphericallens 1006 b.

The spherical lens 1006 a has a first surface with a radius of curvatureof 125 mm, a second surface with a radius of curvature of 77 mm, and athickness of 10 mm, while the spherical lens 1006 b has a first surfacewith a radius of curvature of 97 mm, a second surface with a radius ofcurvature of −200 mm, and a thickness of 20 mm. The distance between asecond surface of the spherical lens 1006 a and a first surface of thespherical lens 1006 b is 5.5 mm. If the homogeneity of the beam spot onthe irradiation surface 1007 is not required that much or if an F-number(F=lens focal length/incidence pupil diameter) of the doublet lens isvery large, a singlet lens may be used alternatively.

Thus, a square beam spot with homogeneous energy distribution having alength of 700 μm on each side can be formed on the irradiation surface1007 by using the optical system shown in FIGS. 10A and 10B. The laseroscillator used in combination with the beam homogenizer of the presentinvention preferably has high output power and a wavelength range whichis sufficiently absorbed in a semiconductor film. In the case of using asilicon film as a semiconductor film, the wavelength of a laser beamemitted from a laser oscillator is preferably 600 nm or shorter inconsideration of an absorption ratio.

As a laser oscillator emitting such a laser beam, for example, anexcimer laser, a YAG laser (harmonic), a glass laser (harmonic), a YVO₄laser (harmonic), a YLF laser (harmonic), an Ar laser, a GdVO₄ laser(harmonic), or a Ti: sapphire laser (harmonic) is given. In addition tothese lasers, other lasers may be used. A laser beam may be convertedinto a harmonic with a wavelength of 600 nm or shorter by using a knownnon-linear optical element. The optical system of the present inventionmay be used in the air or under a nitrogen or argon atmosphere in orderto suppress breakdown or damage of a lens surface due to a laser beamwith high energy.

This embodiment has shown an example of using a spherical planoconvexlens and a spherical planoconcave lens as the elements constituting thefly-eye lens; however, a lens with both of its first and second surfacescurved or a lens with different curvature in the first and seconddirections may be used as well. If the lens with different curvature inthe first and second directions is used, in the case of using aspherical lens as a condensing lens, the beam spot to be formed becomesrectangular. If a rectangular beam with an aspect ratio of 1 or more isto be formed, a toric lens or a crossed cylindrical lens with differentcurvature in the first and second directions may be used as thecondensing lens instead of the spherical lens, and a cylindrical lensfor condensing the beam only in the first direction and a cylindricallens for condensing the beam only in the second direction may bearranged.

Moreover, although this embodiment shows an example of using the fly-eyelens having curvature on one side, one fly-eye lens with both of thefirst and second surfaces having curvature can be used instead of thefly-eye lenses 1003 a and 1003 b similarly to Embodiment 3. Even in thiscase, it is possible to shorten the optical path length as compared withthe case of using, instead of the fly-eye lens with its oppositesurfaces having curvature, a planoconvex fly-eye lens having the samefocal length as the fly-eye lens with its opposite surfaces havingcurvature similarly to Embodiment 3. Moreover, a similar advantageouseffect can be obtained even if the fly-eye lenses 1004 a and 1004 b arereplaced by one fly-eye lens with its opposite surfaces havingcurvature.

Further, the fly-eye lenses 1003 a and 1003 b can be replaced by onefly-eye lens with its opposite surfaces having curvature and the fly-eyelenses 1004 a and 1004 b can be replaced by one fly-eye lens with itsopposite surfaces having curvature. In such a case, an effect ofshortening an optical path can be obtained by replacing the front sideand back side lenses. That is to say, an effect of shortening an opticalpath in the case of using a front side array lens of an optical pathshortened type and a back side array lens of an optical path shortenedtype can be obtained.

This application is based on Japanese Patent Application serial no.2005-106392 filed in Japan Patent Office on Apr. 1, 2005, the entirecontents of which are hereby incorporated by reference. The laserirradiation apparatus disclosed in this specification can be used forvarious purposes, for example, for crystallizing a semiconductor film.The semiconductor film crystallized in accordance with the laserirradiation of the present invention can be used as an active layer of asemiconductor device such as a thin film transistor. As an example, sucha thin film transistor may be manufactured in accordance with the methoddisclosed in U.S. Patent Application Publication No. 2004/0256618 A1 byImai et al. although not limited thereto. An entire disclosure of U.S.Patent Application Publication No. 2004/0256618 A1 is hereinincorporated by reference.

1. A beam homogenizer comprising: a first lens system of an optical pathshortened type; a second lens system of an optical path shortened type;and a condensing lens, wherein a first principal point of the secondlens system is positioned on a beam emission side of the second lenssystem, wherein a second principal point of the first lens system ispositioned on a beam incidence side, and wherein the first lens systemand the second lens system and the condensing lens are in a same opticalpath.
 2. A beam homogenizer comprising: a first lens system of anoptical path shortened type; a second lens system of an optical pathunshortened type; and a condensing lens; wherein a second principalpoint of the first lens system is positioned on a beam incidence side,and wherein the first lens system and the second lens system and thecondensing lens are in a same optical path.
 3. A beam homogenizercomprising: a first lens system of an optical path unshortened type; asecond lens system of an optical path shortened type; and a condensinglens; wherein a first principal point of the second lens system ispositioned on a beam emission side, and wherein the first lens systemand the second lens system and the condensing lens are in a same opticalpath.
 4. A method of manufacturing a semiconductor device comprising:forming a semiconductor film over a substrate; crystallizing thesemiconductor film by irradiating a laser beam homogenized by a beamhomogenizer; forming semiconductor layer by patterning the semiconductorfilm; forming a gate electrode over the semiconductor layer with a gateinsulating film interposed therebetween; and doping an impurity elementto the semiconductor layer selectively to form a source region and adrain region in the semiconductor layer; wherein the beam homogenizercomprises: a first lens system of an optical path shortened type; asecond lens system of an optical path shortened type; and a condensinglens, wherein a first principal point of the second lens system ispositioned on a beam emission side of the second lens system, wherein asecond principal point of the first lens system is positioned on a beamincidence side, and wherein the first lens system and the second lenssystem and the condensing lens are arranged along a same optical path inseries.
 5. A method of manufacturing a semiconductor device comprising:forming a semiconductor film over a substrate; crystallizing thesemiconductor film by irradiating a laser beam homogenized by a beamhomogenizer; forming semiconductor layer by patterning the semiconductorfilm; forming a gate electrode over the semiconductor layer with a gateinsulating film interposed therebetween; and doping an impurity elementto the semiconductor layer selectively to form a source region and adrain region in the semiconductor layer; wherein the beam homogenizercomprises: a first lens system of an optical path shortened type; asecond lens system of an optical path unshortened type; and a condensinglens; wherein a second principal point of the first lens system ispositioned on a beam incidence side, and wherein the first lens systemand the second lens system and the condensing lens are arranged along asame optical path in series.
 6. A method of manufacturing asemiconductor device comprising: forming a semiconductor film over asubstrate; crystallizing the semiconductor film by irradiating a laserbeam homogenized by a beam homogenizer; forming semiconductor layer bypatterning the semiconductor film; forming a gate electrode over thesemiconductor layer with a gate insulating film interposed therebetween;and doping an impurity element to the semiconductor layer selectively toform a source region and a drain region in the semiconductor layer;wherein the beam homogenizer comprises: a first lens system of anoptical path unshortened type; a second lens system of an optical pathshortened type; and a condensing lens; wherein a first principal pointof the second lens system is positioned on a beam emission side, andwherein the first lens system and the second lens system and thecondensing lens are arranged along a same optical path in series.
 7. Thebeam homogenizer according to claim 1, wherein a distance between thesecond principal point of the first lens system and the first principalpoint of the second lens system is equal to a focal length of the secondlens system.
 8. The beam homogenizer according to claim 2, wherein adistance between the second principal point of the first lens system andthe first principal point of the second lens system is equal to a focallength of the second lens system.
 9. The beam homogenizer according toclaim 3, wherein a distance between the second principal point of thefirst lens system and the first principal point of the second lenssystem is equal to a focal length of the second lens system.
 10. Thebeam homogenizer according to claim 1, wherein the first lens systemincludes a first combined array lens, and the second lens systemincludes a second combined array lens.
 11. The beam homogenizeraccording to claim 1, wherein the first lens system includes a firstarray lens having curved surfaces on opposite sides thereof and thesecond lens system includes a second array lens having curved surfaceson opposite sides thereof.
 12. The beam homogenizer according to claim2, wherein the first lens system includes a combined array lens and thesecond lens system includes a first array lens.
 13. The beam homogenizeraccording to claim 2, wherein the first lens system includes a firstarray lens having curved surfaces on opposite sides thereof.
 14. Thebeam homogenizer according to claim 3, wherein the first lens systemincludes a first array lens and the second lens system includes acombined array lens.
 15. The beam homogenizer according to claim 3,wherein the second lens system includes a first array lens having curvedsurfaces on opposite sides thereof.
 16. The beam homogenizer accordingto claim 10, wherein the first combined array lens includes a firstarray lens and a second array lens, wherein the second combined arraylens includes a third array lens and a fourth array lens, wherein acurved surface of the first and fourth array lenses are convex and acurved surface of the second and third array lenses are concave, andwherein each of the first array lens and the second array lens is acylindrical lens array.
 17. The beam homogenizer according to claim 12,wherein the combined array lens includes a second array lens and a thirdarray lens, wherein a curved surface of the second array lens is convexand a curved surface of the third array lens is concave, and whereineach of the first, second and third array lens is a cylindrical lensarray.
 18. The beam homogenizer according to claim 14, wherein thecombined array lens includes a second array lens and a third array lens,wherein a curved surface of the second array lens is concave and acurved surface of the third array lens is convex, and wherein each ofthe first, second and third array lens is a cylindrical lens array. 19.The beam homogenizer according to claim 10, wherein the first combinedarray lens includes a first array lens and a second array lens, whereinthe second combined array lens includes a third array lens and a fourtharray lens, wherein a curved surface of the first and fourth arraylenses are convex and a curved surface of the second and third arraylenses are concave, and wherein each of the first array lens and thesecond array lens is a fly-eye lens.
 20. The beam homogenizer accordingto claim 12, wherein the combined array lens includes a second arraylens and a third array lens, wherein a curved surface of the secondarray lens is convex and a curved surface of the third array lens isconcave, and wherein each of the first, second and third array lens is afly-eye lens.
 21. The beam homogenizer according to claim 14, whereinthe combined array lens includes a second array lens and a third arraylens, wherein a curved surface of the second array lens is convex and acurved surface of the third array lens is concave, and wherein each ofthe first, second and third array lens is a fly-eye lens.
 22. The beamhomogenizer according to claim 11, wherein the first array lens has afirst curved surface and a second curved surface on opposite sidesthereof, wherein the first curved surface is convex and the secondcurved surface is concave, wherein the second array lens has a thirdcurved surface and a fourth curved surface on its opposite sides,wherein the third curved surface is concave and the fourth curvedsurface is convex, wherein the second curved surface of the first arraylens and the third curved surface of the second array lens are facedeach other, and wherein each of the first and the second array lens iscylindrical lens array.
 23. The beam homogenizer according to claim 13,wherein the first array lens has a first curved surface and a secondcurved surface on opposite sides thereof, wherein the first curvedsurface is convex and the second curved surface is concave, wherein thesecond lens system includes a second array lens having a curved surfaceon one side thereof, and wherein each of the first array lens and thesecond array lens is a cylindrical lens array.
 24. The beam homogenizeraccording to claim 15, wherein the first array lens has a first curvedsurface and a second curved surface on opposite sides thereof, whereinthe first curved surface is concave and the second curved surface isconvex, wherein the first lens system includes a second array lenshaving a curved surface on one side thereof, and wherein each of thefirst array lens and the second array lens is a cylindrical lens array.25. The beam homogenizer according to claim 10, wherein the first andthe second combined array lens include two or more array lenses, andeach array lens is a cylindrical lens array or a fly-eye lens.
 26. Thebeam homogenizer according to claim 11, wherein the first array lens andthe second array lens are any one of a cylindrical lens array, a fly-eyelens, and a crossed cylindrical lens array.
 27. The beam homogenizeraccording to claim 12, wherein the first array lens is a composite arraylens or an incomposite array lens.
 28. The beam homogenizer according toclaim 14, wherein the first array lens is a composite array lens or anincomposite array lens.
 29. The beam homogenizer according to claim 27,wherein the incomposite array lens is a cylindrical lens array or afly-eye lens.
 30. The beam homogenizer according to claim 28, whereinthe incomposite array lens is a cylindrical lens array or a fly-eyelens.
 31. The beam homogenizer according to claim 1, wherein thecondensing lens is a cylindrical lens, a toric lens, or a crossedcylindrical lens.
 32. The beam homogenizer according to claim 2, whereinthe condensing lens is a cylindrical lens, a toric lens, or a crossedcylindrical lens.
 33. The beam homogenizer according to claim 3, whereinthe condensing lens is a cylindrical lens, a toric lens, or a crossedcylindrical lens.
 34. The beam homogenizer according to claim 1, whereinthe beam homogenizer is incorporated into a laser irradiation apparatuscomprising a laser oscillator and a stage, wherein the beam homogenizerhomogenizes energy density distribution in one direction of a short-sideand a long-side direction and then homogenizes energy densitydistribution in the other direction, wherein an irradiation surface towhich an irradiation beam with its energy density distributionhomogenized in the both directions is projected is set on the stage, andwherein the laser oscillator emits a laser beam.
 35. The beamhomogenizer according to claim 2, wherein the beam homogenizer isincorporated into a laser irradiation apparatus comprising a laseroscillator and a stage, wherein the beam homogenizer homogenizes energydensity distribution in one direction of a short-side and a long-sidedirection and then homogenizes energy density distribution in the otherdirection, wherein an irradiation surface to which an irradiation beamwith its energy density distribution homogenized in the both directionsis projected is set on the stage, and wherein the laser oscillator emitsa laser beam.
 36. The beam homogenizer according to claim 3, wherein thebeam homogenizer is incorporated into a laser irradiation apparatuscomprising a laser oscillator and a stage, wherein the beam homogenizerhomogenizes energy density distribution in one direction of a short-sideand a long-side direction and then homogenizes energy densitydistribution in the other direction, wherein an irradiation surface towhich an irradiation beam with its energy density distributionhomogenized in the both directions is projected is set on the stage, andwherein the laser oscillator emits a laser beam.
 37. The beamhomogenizer according to claim 1, wherein the beam homogenizer isincorporated into a laser irradiation apparatus comprising a laseroscillator and a stage, wherein the beam homogenizer homogenizes energydensity distribution in a short-side direction and a long-side directionwherein an irradiation surface to which an irradiation beam with itsenergy density distribution homogenized in the both directions isprojected is set on the stage, and wherein the laser oscillator emits alaser beam.
 38. The beam homogenizer according to claim 2, wherein thebeam homogenizer is incorporated into a laser irradiation apparatuscomprising a laser oscillator and a stage, wherein the beam homogenizerhomogenizes energy density distribution in a short-side direction and along-side direction wherein an irradiation surface to which anirradiation beam with its energy density distribution homogenized in theboth directions is projected is set on the stage, and wherein the laseroscillator emits a laser beam.
 39. The beam homogenizer according toclaim 3, wherein the beam homogenizer is incorporated into a laserirradiation apparatus comprising a laser oscillator and a stage, whereinthe beam homogenizer homogenizes energy density distribution in ashort-side direction and a long-side direction, wherein an irradiationsurface to which an irradiation beam with its energy densitydistribution homogenized in the both directions is projected is set onthe stage, and wherein the laser oscillator emits a laser beam.
 40. Thelaser irradiation apparatus according to claim 34, wherein the laseroscillator is at least one selected from the group consisting of anexcimer laser, a YAG laser, a glass laser, a YVO₄ laser, a YLF laser, anAr laser, a GdVO₄ laser and a Ti:sapphire laser.
 41. The laserirradiation apparatus according to claim 35, wherein the laseroscillator is at least one selected from the group consisting of anexcimer laser, a YAG laser, a glass laser, a YVO₄ laser, a YLF laser, anAr laser, a GdVO₄ laser and a Ti:sapphire laser.
 42. The laserirradiation apparatus according to claim 36, wherein the laseroscillator is at least one selected from the group consisting of anexcimer laser, a YAG laser, a glass laser, a YVO₄ laser, a YLF laser, anAr laser, a GdVO₄ laser and a Ti:sapphire laser.
 43. The laserirradiation apparatus according to claim 37, wherein the laseroscillator is at least one selected from the group consisting of anexcimer laser, a YAG laser, a glass laser, a YVO₄ laser, a YLF laser, anAr laser, a GdVO₄ laser and a Ti:sapphire laser.
 44. The laserirradiation apparatus according to claim 38, wherein the laseroscillator is at least one selected from the group consisting of anexcimer laser, a YAG laser, a glass laser, a YVO₄ laser, a YLF laser, anAr laser, a GdVO₄ laser and a Ti:sapphire laser.
 45. The laserirradiation apparatus according to claim 39, wherein the laseroscillator is at least one selected from the group consisting of anexcimer laser, a YAG laser, a glass laser, a YVO₄ laser, a YLF laser, anAr laser, a GdVO₄ laser and a Ti:sapphire laser.